1. Field of the Invention
The present invention relates generally to marine seismology, in which a moving ship generates seismic waves and detects reflections. Still more particularly, the invention relates to processing the reflected seismic waves to correct for the motion of the ship.
2. Background of the Invention
The field of seismology focuses on the use of artificially generated elastic waves to locate mineral deposits such as hydrocarbons, ores, water, and geothermal reservoirs. Seismology is also used for archaeological purposes and to obtain geological information for engineering. Exploration seismology provides data that, when used in conjunction with other available geophysical, borehole, and geological data, can provide information about the structure and distribution of rock types and their contents.
Most oil companies rely on interpretation of seismic data for selecting the sites in which to invest in drilling exploratory oil wells. Despite the fact that seismic data is used to map geological structures rather than finding petroleum directly, the gathering of seismic data has become a vital part of selecting the site of an exploratory and development well. Experience has shown that the use of seismic data greatly improves the likelihood of a successful venture.
Seismic data acquisition is routinely performed both on land and at sea. At sea, seismic ships deploy a streamer or cable behind the ship as the ship moves forward. The streamer includes multiple receivers in a configuration generally as shown in FIG. 1. Streamer 110 trails behind ship 100 which moves in the direction of the arrow 102. As shown in FIG. 1, source 112 is also towed behind ship 100. Source 112 and receivers 114 typically deploy below the surface of the ocean 104. Streamer 110 also includes electrical or fiber-optic cabling for interconnecting receivers 114, and the seismic equipment on ship 100. Streamers are usually constructed in sections 25 to 100 meters in length and include groups of up to 35 or more uniformly spaced receivers. The streamers may be several miles long and often a seismic ship trails multiple streamers to increase the amount of seismic data collected. Data is digitized near the receivers 114 and is transmitted to the ship 100 through the cabling at rates of 7 (or more) million bits of data per second. Processing equipment aboard the ship controls the operation of the trailing source and receivers and processes the acquired data.
Seismic techniques estimate the distance between the ocean surface 104 and subsurface structures, such as structure 106 which lies below the ocean floor 108. By estimating the distance to a subsurface structure, the geometry or topography of the structure can be determined. Certain topographical features are indicative of oil and/or gas reservoirs.
To determine the distance to subsurface structure 106, source 112 emits seismic waves 116 which reflect off subsurface structure 106. The reflected waves are sensed by receivers 114. By determining the length of time that the seismic waves 116 took to travel from source 112 to subsurface structure 106 to receivers 114, an estimate of the distance to subsurface structure 106 can be obtained.
The receivers used in marine seismology are commonly referred to as hydrophones, or marine pressure phones, and are usually constructed using a piezoelectric transducer. Synthetic piezoelectric materials, such as barium zirconate, barium titanate, or lead mataniobate, are generally used. A sheet of piezoelectric material develops a voltage difference between opposite faces when subjected to mechanical bending. Thin electroplating on these surfaces allows an electrical connection to be made to the device so that this voltage can be measured. The voltage is proportional to the amount of mechanical bending or pressure change experienced by the receiver as resulting from seismic energy propagating through the water. Various types of hydrophones are available such as disk hydrophones and cylindrical hydrophones.
Two major types of seismic sources are used to generate seismic waves for the seismic measurements. The first major source type comprises an impulsive source which generates a high energy, short time duration impulse. The time between emission of the impulse from the source and detection of the reflected impulse by a receiver is used to determine the distance to the subsurface structure under investigation. The impulsive source and the associated data acquisition and processing system are relatively simple. However, the intensity of energy required by seismic techniques using impulsive sources may, in some situations, be harmful to marine life in the immediate vicinity of source 112.
The environmental concerns associated with impulsive sources has lead to the use of another type of seismic source which generates a lower magnitude, extended duration, vibratory energy. The measurement technique which uses such a source is referred to as the marine vibratory seismic ("MVS") technique. Rather than imparting a high magnitude pressure pulse into the ocean in a very short time period, vibratory sources emit lower amplitude pressure waves over a time period typically between 5 and 7 seconds, but longer time periods are also possible. The frequency of the vibrating source varies from about 5 to 150 Hz, although the specific low and high frequencies differ from system to system. The frequency of the source may vary linearly or nonlinearly with respect to time. Such a frequency variation pattern is commonly called a "frequency sweep". The frequency sweep is thus between 5 and 150 Hz in frequency and 5 to 7 seconds in duration. The magnitude of the seismic wave oscillations may vary or remain at a constant amplitude. The amplitude of the oscillations, however, are much lower than the magnitude of impulsive sources and thus, there are fewer environmental concerns with the MVS seismic technique.
Seismic ships must move forward while seismic measurements are being recorded for many reasons. The hydrophones 114, along with connecting wires and stress members provided on the streamers, are typically placed inside a neoprene tube (not shown in FIG. 1) 2.5-5 inches in diameter. The tube is then filled with sufficient lighter-than-water liquid to make the streamer neutrally buoyant. When the streamer is moving forward, a diverter 118 pulls the streamer 114 out to an appropriate operating width. Depth controllers (not shown) are fastened to the streamer at various places along its length. These devices sense the hydrostatic pressure and tilt bird wings so that the flow of water over them raises or lowers the streamer to the desired depth. The depth that the controllers seek to maintain can be controlled by a signal sent through the streamer cabling and thus the depth can be changed as desired.
For the streamer's depth control system to function effectively, the ship 100 must travel forward at a speed through the water of approximately four knots. Additionally, since streamer 110 is normally a flexible cable, the ship must move forward to maintain a desired fixed separation between the sources 112 and streamers 110. The spacing between sources and streamers is important in the marine seismology and must not vary while seismic measurement are made.
Forward motion is also necessary for the operation of diverter(s) 118. When seismic ships deploy multiple streamers, the diverters are used to provide fixed separation between streamers. These diverters force the streamers laterally as the boat moves forward. Without the diverters, the streamers may become entangled. The relative velocity of the water around the diverters and the angle of attack determine the amount of separation between streamers.
Forward motion of the ship is additionally beneficial because it allows the seismic ships to cover as much ocean surface as possible each day. For these reasons and others, seismic ships must move forward while taking measurements and the forward speed must be reasonably constant. Typical ship speed is approximately 2-3 meters per second. Because the streamer is deployed behind the ship, the source and receivers also move at approximately 2.5 meters per second.
Marine seismic measurements can also be made using a technique called "on-bottom cable" (OBC) in which a ship lays one or more cables containing hydrophones and geophones on the ocean floor. This ship remains stationary and records data while collecting seismic data. A second ship containing sources moves parallel, or at some other angle, to the cables. In the OBC technique, the receivers do not move, but the sources are moving and thus, the acquired data may be distorted. Further, in special circumstances, some of the receivers can be on land. Although OBC is generally more expensive than towed marine seismic measurements, OBC may be necessary if land obstructions, such as an island, are located in the survey area.
Although ship motion is necessary as described above, the motion distorts or "smears" the acquired seismic data. Broadly, smearing results from the fact that the ship, and thus the sources and receivers, move while data collection takes place. Data smearing in a MVS system includes significant contributions from both receiver and source motion. Thus, the MVS-acquired data should be corrected for both receiver and source motion. Previous attempts to correct for ship motion have relied on a constant-velocity assumption that is inexact for most geological structures.
It would be advantageous to provide a practical seismic system for use in marine applications that can correct the data for the motion of the ship. Such a system preferably would correct for both receiver and source motion and do so in a cost effective manner.